To induce antigen-specific T cell activation and clonal expansion, two signals provided by antigen-presenting cells (APCs) must be delivered to the surface of resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med. 165, 302-319; Mueller, D. L., et al. (1990) J. Immunol. 144, 3701-3709; Williams, I. R. and Unanue, E. R. (1990) J. Immunol 145, 85-93). The first signal, which confers specificity to the immune response, is mediated via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Schwartz, R. H. (1990) Science 248, 1349-1356). Costimulation is neither antigen-specific, nor MHC restricted and is thought to be provided by one or more distinct cell surface molecules expressed by APCs (Jenkins, M. K., et al. (1988) J. Immunol 140, 3324-3330; Linsley, P. S., et al. (1991) J. Exp. Med. 173, 721-730; Gimmi, C. D., et al., (1991) Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Young, J. W., et al. (1992) J. Clin. Invest. 90, 229-237; Koulova, L., et al. (1991) J. Exp. Med. 173, 759-762; Reiser, H., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 271-275; van-Seventer, G. A., et al. (1990) J. Immunol. 144, 4579-4586; LaSalle, J. M., et al., (1991) J. Immunol. 147, 774-80; Dustin, M. I., et al., (1989) J. Exp. Med. 169, 503; Armitage, R. J., et al. (1992) Nature 357, 80-82; Liu, Y., et al. (1992) J. Exp. Med. 175, 437-445).
Considerable evidence suggests that the B7 protein, expressed on APCs, is one such critical costimulatory molecule (Linsley, P. S., et al., (1991) J. Exp. Med. 173, 721-730; Gimmi, C. D., et al., (1991) Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Koulova, L., et al., (1991) J. Exp. Med. 173, 759-762; Reiser, H., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 271-275; Linsley, P. S. et al. (1990) Proc. Natl. Acad. Sci. USA. 87, 5031-5035; Freeman, G. J. et al. (1991) J. Exp. Med. 174, 625-631.). B7 is the counter-receptor for two ligands expressed on T lymphocytes. The first ligand, termed CD28, is constitutively expressed on resting T cells and increases after activation. After signaling through the T cell receptor, ligation of CD28 induces T cells to proliferate and secrete IL-2 (Linsley, P. S., et al. (1991) J. Exp. Med. 173, 721-730; Gimmi, C. D., et al. (1991) Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Thompson, C. B., et al. (1989) Proc. Natl. Acad. Sci. USA. 86, 1333-1337; June, C. H., et al. (1990) Immunol. Today. 11, 211-6; Harding, F. A., et al. (1992) Nature. 356, 607-609.). The second ligand, termed CTLA4 is homologous to CD28 but is not expressed on resting T cells and appears following T cell activation (Brunet, J. F., et al., (1987) Nature 328, 267-270). DNA sequences encoding the human and murine CTLA4 protein are described in Dariavach, et al. (1988) Eur. J. Immunol. 18(12), 1901-1905; Brunet, J. F., et al. (1987) supra; Brunet, J. F. et al. (1988) Immunol. Rev. 103:21-36; and Freeman, G. J., et al. (1992) J. Immunol 149, 3795-3801. Although B7 has a higher affinity for CTLA4 than for CD28 (Linsley, P. S., et al., (1991) J. Exp. Med. 174, 561-569), the function of CTLA4 is still unknown.
The importance of the B7:CD28/CTLA4 costimulatory pathway has been demonstrated in vitro and in several in vivo model systems. Blockade of this costimulatory pathway results in the development of antigen specific tolerance in murine and humans systems (Harding, F. A., et al. (1992) Nature. 356, 607-609; Lenschow, D. J., et al. (1992) Science. 257, 789-792; Turka, L. A., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 11102-11105; Gimmi, C. D., et al. (1993) Proc. Natl. Acad. Sci USA 90, 6586-6590; Boussiotis, V., et al. (1993) J. Exp. Med. 178, 1753-1763). Conversely, expression of B7 by B7 negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L., et al. (1992) Cell 71, 1093-1102; Townsend, S. E. and Allison, J. P. (1993) Science 259, 368-370; Baskar, S., et al. (1993) Proc. Natl. Acad. Sci. 90, 5687-5690.). Therefore, manipulation of the B7:CD28/CTLA4 pathway offers great potential to stimulate or suppress immune responses in humans.